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A Generalized Geologic Map of Delaware

This map was created from published 1:100,000-scale geologic maps of New Castle and Kent Counties, and the most current knowledge about the surficial geology of Sussex County. Sussex County was compiled from published 1:24,000-scale geologic maps of various quadrangles and recent fieldwork. (The current version of Sussex County seen here is the same as published in RI76.)

The Delaware Geological Survey (DGS) published the surficial geology of the state of Delaware at a scale of 1:100,000 for New Castle and Kent counties (Ramsey, 2005, 2007). Maps at this scale are useful for viewing general geologic framework on a county-wide basis, determining the geology of watersheds, and recognizing the relationship of geology to county-wide environmental or land-use issues. These maps, when combined with subsurface geologic information, provide a basis for locating water supplies, mapping groundwater recharge areas, and protecting ground and surface water. Geologic maps are also used to identify geologic hazards, such as flood-prone areas, to identify sand and gravel resources, and for supporting state, county, and local land-use planning decisions. Portions of Sussex County have previously been mapped at a scale of 1:24,000.

Field work is in progress in eastern Sussex County and will be complete in the near future.

GIS Methods Used to Create this Map

Heads-up digitizing was performed in ArcGIS from maps on which the geologist drew geologic unit boundaries. Geologic unit boundaries are determined through field interpretation of well and borehole data, aerial photographs, as well as contours from LiDAR. Data compilation methods included merging existing geologic attributes (Ramsey, 1993, 2001, and 2003; Andres and Ramsey, 1995; Schenck et al., 2000), clipping polygons to the state boundary, and extracting and merging the swamp/marsh attribute from the USGS National Hydrography Dataset (NHD). Existing geologic digital data and the NHD are both mapped at a scale of 1:24,000. These areas were designed to be viewed at a scale of 1:100,000 (polygons with an area less than 25,000 square meters were deleted).

Base map data (not shown) were compiled for geographic location and editing purposes. Datasets included hydrography, transportation and boundaries for the state of Delaware (http://datamil.delaware.gov). Hydrography data were also generalized to a scale of 1:100,000 using the same method for generalizing the geologic data. A few deleted hydrography polygons were reintegrated into the dataset since they connected stream line data.